This invention relates in general to oxygen sensors and in particular to the utilization of an oxygen sensor with an internal combustion engine.
Internal combustion engines can use Oxygen (O2) sensors to monitor the Air to Fuel Ratio (AFR) and ensure efficient combustion. Ideally, an AFR would be utilized to provide a stoichiometric combustion in which the fuel is completely burned. Stoichiometric combustion for gasoline requires a weight ratio of 14.7 parts of air to one part of fuel.
Referring now to FIG. 1, there is shown a schematic diagram illustrating a prior art internal combustion engine control system 10. The system 10 includes a narrow band O2 sensor 12 that outputs a voltage indicating the presence of oxygen in the exhaust. The sensor output is sent to an Engine Control Unit (ECU) 14 that is responsive to the sensor output to modify a Pulse Width Modulated (PWM) control signal having a variable pulse width and/or duration. The PWM control signal is, in turn, sent to the engine fuel injectors 16. Narrow band sensors used with the ECU output a voltage transitioning between 0 and 1 volts over a narrow range of AFR near 14.7, as illustrated by the graph to the left in FIG. 2. A rich mixture with an AFR just below 14.7 will output a voltage near 1 volt, and a lean mixture with an AFR just above 14.7 will output a voltage approaching 0 volts, as illustrated by the graph to right in FIG. 2. The graphs shown in FIG. 2 illustrate the AFR output for an idealized O2 sensor output, while the graphs shown in FIG. 3 illustrate the AFR output for a typical O2 actual sensor output. The ECU 14 will add fuel to the air-fuel mixture if there is a lean condition, and it will subtract fuel from the air-fuel mixture if there is a rich condition by varying the pulse width, or the duty cycle, of the PWM signal sent to the fuel injectors 16. Therefore, the ECU continuously controls the engine fuel injectors so the AFR is maintained close to the ideal stoichiometric AFR (AFRStoich).
It will be noted that most technical books and articles discuss an “excess air factor,” or lambda (λ), instead of AFR, with λ, being the ratio of the actual AFR to the stoichiometric AFR. Thus, a λ=1.0 represents stoichiometric combustion. Lambda is used because various fuels combine differently, and a strict weight ratio of 14.7 parts of air to one part of fuel is applicable only for a specific fuel. When λ is utilized, a rich condition has λ<1.0, while a lean condition has λ>1.0. However, AFR is used in calculations to determine an actual quantity of fuel. For a given intake stroke, a finite quantity of air is acquired. Thus, fuel volume is utilized as the only variable to obtain a different AFR.
In the U.S., Europe, and Japan, catalytic after-treatment of engine exhaust gas using a catalytic converter gas has proven to be the only means of complying with the present limits for CO, NO, and HC. Catalytic converters function most effectively when λ=1. Therefore, engine controllers are designed to operate within a narrow range with λ=1.0±0.005.
In order to enhance engine performance, current available systems can control AFR by using a wide band O2 sensor, while still providing a narrow band O2 signal to the ECU, as illustrated by the engine control system 20 shown in FIG. 4. Components shown in FIG. 4 that are the same as components shown in FIG. 1 have the same numerical identifiers. As shown in FIG. 4, both a wide band O2 sensor 22 and a Fuel Injector Controller (FIC) 24 have replaced the narrow band O2 sensor 12 shown in FIG. 1. The FIC 24 is responsive to the output signal received from the wide band O2 sensor 22 to generate a false narrow band signal that is sent to the ECU 14. The false narrow band signal causes the ECU to add or subtract fuel. If a lower AFR is desired, a low narrow band O2 signal is sent to the ECU so that fuel is added. Similarly, if a higher AFR is desired, a high narrow band O2 signal is sent to the ECU so that fuel is subtracted.
The system 20 may encounter problems with newer ECUs, in which sensors are cross checked with other systems. For example, a mass air flow sensor (not shown) can calculate the amount of engine input air, which can be compared to the PWM signal being sent to the fuel injectors 16. With the system 20 shown in FIG. 2, the amount of engine input air will not compare satisfactorily with the output injector pulse width, causing the ECU to generate an error signal. Accordingly, it would be desirable to be able to utilize a wide band O2 sensor with the newer ECUs without an error signal being generated.